1. Field of the Invention
The present invention relates to an apparatus and method for the detection of leaks and qualification of vacuum systems. More specifically, an apparatus and method is provided for detecting gaseous components of air, as well as a trace gas.
2. Background of the Related Art
Vacuum system qualification against leaks is necessary to ensure proper operation of the system. Various methods and apparatus are currently used for leak detection. Typically, vacuum systems and individual components are qualified either by trace gas leak detection or by Rate of Rise (ROR) and/or base pressure (P.sub.B) testing or a combination of these methods.
A trace gas leak detector utilizes a Mass Spectrometer Leak Detector (MSLD) capable of detecting a gas, such as helium, in a test object, such as a vacuum system. The MSLD comprises a spectrometer tube to measure the partial pressure of the trace gas in a vacuum system. Electrons produced by a hot filament in the spectrometer tube travel toward a positive grid. In transit, some of the electrons collide with the gas molecules, thereby creating ions. A magnetic field is employed to deflect the various ions according to their mass-to-charge ratio allowing only the desired trace gas ions to pass through the field and arrive at a collector. The trace gas ions then strike the collector and raise the potential of the collector in proportion to their arrival rate. This potential increase is measured by an electrometer amplifier and displayed on the output meter. The readout is proportional to the total pressure in the spectrometer tube. The higher the pressure in the tube, the more trace gas molecules are present, and the more trace gas ions are created.
An exemplary MSLD 10 is shown in FIG. 1 coupled to a test object 11, such as a vacuum chamber. In general, the MSLD 10 consists of a manifold 12 having a series of valves and a manometer 32 disposed therein, a magnetic sector spectrometer tube 14 sensitive to helium (or other trace gas), a high vacuum pump 16, such as a turbomolecular pump, to maintain an adequately low operating pressure in the spectrometer tube 14, a mechanical pump 18 such as a roughing pump, to evacuate the component or system to be tested, a power supply 20, and an output unit 22 comprising an amplifier and readout instrumentation, such as a meter, to monitor an output signal from the spectrometer tube 14. The test object 11 is coupled to an inlet end 24 of the manifold 12 and selectively communicates with either the mechanical pump 18 or the high vacuum pump 16 according to the valve sequencing during operation. A manual shut-off valve 30 is disposed between the test object 11 and the MSLD 10 to selectively isolate the two components from one another.
Initially, the roughing valve 26 is opened and the test object 11 is pumped down by the roughing pump 18. The pressure in the test object 11 is measured by the manometer 32. Once a base pressure is reached in the test object 11, the test valve 28 is opened to provide fluid communication with the high vacuum pump 16, thus allowing gas molecules from the test object 11 to flow into the spectrometer tube 14. A trace gas, such as helium, is then sprayed around the exterior of the test object 11. If vacuum leaks are present, the helium is drawn into the test object 11 at the location of the leaks. The spectrometer tube 14 then measures the partial pressure of the trace gas and generates a signal which is received by the amplifier and displayed on the output meter.
The foregoing apparatus and method is currently accepted in the industry as an excellent leak check tool for isolating leaks. Further, helium leak checks are capable of detecting very small leaks on the order of 10.sup.-8 to 10.sup.-10 sccs. However, helium leak detection is an extremely sensitive technique requiring the technician to apply a calibrated amount of helium, at a specified distance, moving at a specified rate across the unit being tested. A complex test object, such as a typical semiconductor processing cluster tool for example, comprises thousands of sealing surfaces and welds of varying types. Successful helium testing requires uniform testing methods at each location on the cluster tool. Consequently, the accuracy may vary by a factor of ten for a single operator and a factor of twenty to one hundred between different operators. In some cases, the leaks may be missed altogether if the appropriate location is not sprayed with helium.
In order to avoid the inaccuracy of helium leak detection and in an attempt to further automate leak detection, the industry has adopted various very rough methods of gross leak detection. Gross leak detection implies testing techniques adapted merely to indicate the presence of a leak in the device under test without locating the precise location of the leak. Such methods include the use of trace gas environments testing, the Rate of Rise (ROR) method and the P.sub.B (base pressure) method.
A trace gas environment test involves establishing an enclosure around a test object and subjecting the enclosure to a trace gas. Typically, the enclosure is provided using an inflatable bag that is disposed around the test object to seal the object from ambient conditions. A trace gas, such as helium, is then introduced into the bag to create a helium-rich environment around the test object. The test object is then pumped to a sub-atmospheric condition and a trace gas detector is used to monitor the presence of the trace gas in the test object. If the trace gas is detected in the device being tested, a leak is present.
The P.sub.B and ROR methods both use conventional manometers available on a vacuum chamber, such as capacitance manometers and ion gauges, to determine the existence of a leak. Both methods are total pressure tests, i.e., the methods observe the total pressure of the system rather than characterizing the component partial pressures which make up the total pressure.
The P.sub.B method involves pumping a chamber down to determine the lowest achievable pressure which is then checked against an acceptable pass/fail P.sub.B value. If the lowest achievable pressure is less than or equal to the predetermined pass/fail P.sub.B value, the chamber is considered qualified and sufficiently leak free. Conversely, if the lowest achievable pressure is greater than a predetermined pass/fail P.sub.B value, the chamber fails the test and must be reworked to eliminate any leaks.
The ROR method involves pumping a chamber down to a desired base pressure, P.sub.B, and then isolating the chamber from the associated pumping system. The internal chamber pressure change is then observed and checked against an acceptable pass/fail rate. If the rate of rise of the chamber is less than the acceptable rate, the chamber is considered qualified. Conversely, if the rate of rise is greater than the acceptable rate, the chamber fails the test and the chamber must be reworked to eliminate leaks.
The ROR method and the P.sub.B method may be used independently or in combination. Which test is most appropriate is dependent on the system under test. For baked systems, either the P.sub.B or the ROR methods are appropriate. For unbaked medium vacuum systems, e.g., in the millitorr regime, the ROR method is most appropriate. A baked system refers to a system which has been outgassed for a period of time to remove contaminants (e.g., water vapor and oxygen) from the internal chamber surfaces. Typically, baking a system involves pumping the system down to a pressure below the vapor pressures of the contaminants and may also involve heating the system to an elevated temperature to enhance the outgassing.
Each of the foregoing gross leak tests are somewhat limited. The trace gas environment test, for example, requires establishing an artificial environment by means of an inflatable bag. The bag is cumbersome and requires time-consuming efforts to ensure that a sufficiently leak-free enclosure has been established. Further, large quantities of a costly trace gas, such as helium, are required to conduct the test. If the initial test confirms the existence of a leak, the bag must be removed while the leak is repaired. Subsequently, the bag must be disposed over the test object again to confirm that the leak has been sealed. The foregoing procedure is repeated until the test object is qualified. Further, the trace gas environment test merely indicates the existence of a leak without identifying the particular location of the leak. Thus, once a leak is found, an operator indiscriminately reworks each of the fittings and sealing surfaces suspected of leaking without knowledge of which areas of the chamber are, in fact, leaking.
The efficacy of both the P.sub.B test and the ROR test is limited because the base pressure and rate of rise in a chamber are dependent on numerous factors including chamber materials and volume, temperature, and, in particular, outgassing from chamber components. During exposure to ambient conditions, the interior chamber surfaces absorb or collect molecules, such as water vapor. Under vacuum conditions, the molecules are desorbed or outgassed and contribute to the overall chamber pressure. A typical manometer is incapable of characterizing the nature of the leak to determine whether the leak is caused by virtual leaks (internal leaks) due to outgassing, for example, or real leaks (external leaks). Therefore, a leak free chamber may exhibit a failing base pressure or rate of rise due to high levels of outgassing. Consequently, the P.sub.B /ROR methods are highly insensitive and generally valid only for large leaks, e.g., a leak larger than 10.sup.-4 sccs for a ten liter chamber, wherein the partial pressure due to outgassing is minimal compared to the partial pressure of air from external leaks. In general, the larger the volume of the chamber, the larger the leak must be to be detected. While the sensitivity to the P.sub.B /ROR methods may be increased by baking the chamber, baking is a time consuming effort which reduces the throughput of the total system. For example, for a ten liter chamber, baking may require eight hours.
Further, the P.sub.B /ROR methods are gross leak detection methods which monitor the total pressure of the system under test and are incapable of identifying the location of specific leaks. A failed P.sub.B /ROR test merely indicates the possible existence of a leak somewhere in the system. In order to repair the leak, a localizing method, such as conventional helium detection, is needed to pinpoint the precise location of the leak.
By using the P.sub.B /ROR methods in tandem with the more sensitive helium leak check, it is assumed that a higher accuracy rate of leak detection is achieved. Thus, for example, a chamber may be tested by the P.sub.B /ROR methods and subsequently undergo a helium leak check. However, the inherent limitations of both the P.sub.B /ROR methods and the helium leak check prevent even their combined use from achieving sufficient assurance of vacuum integrity. For example, a chamber which passes a helium test of 10.sup.-8 sccs or better may nevertheless fail a P.sub.B /ROR test. This inconsistent result may be due to human error during the helium check and/or high outgassing levels in the chamber indicating a P.sub.B /ROR failure. The dual nature of these leak detection methods results in increased costs and has provided impetus to finding alternative solutions to the problems involved in leak detection.
One solution to the disadvantages of conventional leak detection methods is the use of Residual Gas Analyzers (RGAs) with the P.sub.B /ROR methods. RGAs, such as quadrupoles, are well known in the industry as partial pressure analyzers. Unlike magnetic sector mass spectrometers, discussed above, RGAs are tunable by the operator to detect multiple gases simultaneously and distinctly. As noted above, the P.sub.B /ROR methods merely observe the total pressure of a test object. As such, P.sub.B /ROR tests do not characterize the source(s) contributing to the pressure. No differentiation is made between virtual leaks, due to internal sources such as outgassing (due primarily to water), and real leaks, i.e., holes or openings that allow fluid flow from the outside atmosphere into the vacuum environment. RGAs allow operators to characterize the various gases which contribute to the rate of rise and base pressure of the test object by measuring the individual partial pressures of each gas. Thus, the operator may observe the partial pressures of the primary gases found in air, i.e., oxygen and nitrogen, which indicate a real leak, separately from the partial pressure of water vapor due to outgassing. Performing the P.sub.B /ROR methods with an RGA can increase the sensitivity of standard P.sub.B /ROR leak detection by a factor of roughly one hundred.
In operation, the RGA may be used to measure the partial pressure of the components in air in a vacuum chamber. If unacceptable levels of these components (typically oxygen and/or nitrogen) are detected, a leak is present in the chamber. Subsequently, the RGA is uncoupled from the chamber and the helium MSLD is attached. Helium is then sprayed around the chamber and the helium MSLD measures the partial pressure of helium to pin-point the location of the leak(s).
Although leak testing using an RGA and a helium detector is capable of achieving excellent results, current practice still requires the use of at least two separate components: the RGA and the helium MSLD. The operator is therefore limited to using only one component to the exclusion of the other. Once the initial RGA test determines a leak, the operator must uncouple the RGA and couple the helium MSLD to the test object. After locating one or more leaks, the operator must then uncouple the helium MSLD and reattach the RGA to confirm that no more leaks are present. If some leakage is still detected, the operator must perform the helium leak test again. This cycle is repeated until the test object meets the leak specifications. Such an arrangement results in substantial overhead time and increased costs of testing.
Consequently, one important objective of new leak detection methods and apparatus is automation, i.e., to make the techniques less operator dependent and therefore less prone to operator error. The preferred technique would combine the sensitivity of trace gas leak detection using a MSLD and the simplicity and operator independence of the ROR method. It has been suggested to use a conventional RGA, such as a quadrupole, for helium detection in place of a magnetic sector mass spectrometer tubes. Because quadrupole RGAs are tunable by an operator to detect numerous gases according to atomic mass units (amu), an RGA could be tuned to a trace gas, such as helium. However, trace gas detection requires a high level of sensitivity in order to detect small leaks. Typical quadrupole RGAs are only capable of detecting helium leaks larger than 10.sup.-7 sccs in unbaked systems. A preferred detecting apparatus is sensitive to leaks larger than 10.sup.-9 to 10.sup.-10 sccs in unbaked systems.
Therefore, there remains a need for a method and apparatus for automating leak testing of vacuum systems using a rate of rise and/or base pressure method and a trace gas detector in combination. Preferably, the method and apparatus would allow selection of independent or simultaneous rate of rise testing and trace gas detection with a high degree of sensitivity.